New code for page fault handling introduced in commit efc1c93b7b3e
"[netdrv] i40e/i40evf: Update code to better handle incrementing page count"
(upstream 1793668c3b8c) relies on recenetly introduced __page_frag_cache_drain
function (initially introduced as __page_frag_drain in 8cbd9354f14f "[mm] add
support for releasing multiple instances of a page", upstream 44fdffd70504, and
then updated in 4f21fc318c16 "[mm] rename __page_frag functions to
__page_frag_cache, drop order from drain", upstream 2976db801853), so it's
re-implemented in backport compat header using page_ref_sub_and_test and
__free_pages.
Index: src/drivers/net/ethernet/intel/i40evf/i40e_backport_compat.h
===================================================================
--- src.orig/drivers/net/ethernet/intel/i40evf/i40e_backport_compat.h	2018-03-01 13:52:40.247206589 +0100
+++ src/drivers/net/ethernet/intel/i40evf/i40e_backport_compat.h	2018-03-02 08:32:28.857861042 +0100
@@ -1,6 +1,17 @@
 #ifndef I40EVF_BACKPORT_COMPAT_H
 #define I40EVF_BACKPORT_COMPAT_H
 
+static inline void __page_frag_cache_drain(struct page *page, unsigned int count)
+{
+	VM_BUG_ON_PAGE(page_ref_count(page) == 0, page);
+
+	if (count > 0) {
+		VM_BUG_ON_PAGE(page_ref_sub_and_test(page, count - 1), page);
+
+		__free_pages(page, compound_order(page));
+	}
+}
+
 static inline dma_addr_t dma_map_page_attrs(struct device *dev,
 					    struct page *page,
 					    size_t offset, size_t size,